compel cli: kill --arch option, add --compat

There is no need to support all possible architectures
for "compel cflags" action. In fact, "compel hgen" can
only support the one it was compiled for (with the only
exception of 32-bit mode for x86).

It looks like if we can use a few #ifdefs, there is
no need to specify --arch anymore, let's drop it!

Still, for the x86 32-bit mode we need to introduce
--compat option. Note that "compel hgen" autodetects
32-bit mode for x86 by looking into ELF header, but
in case of "compel clfags" there are no files to look
into, so we need this --compat specified explicitly.

While at it,
 - Makefile: define CONFIG_AARCH64 if building for ARM64
 - fail to compile on unsupported/unspecified ARCH
 - make "compel --help" output a bit more compact

travis-ci: success for More polishing for compel cli
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>